Starch nanocrystal is the remaining acid resistant crystalline portion of starch after acid hydrolysis of the amorphous region, is an organic nanoparticle, and has the following strengths, for example: 1) it is characterized by low cost, wide sources, biorenewability, biodegradability, good biocompatibility and non-toxicity, compared with inorganic nanoparticles, and can be widely used in life science fields, such as food, biology, pharmaceuticals, etc.; 2) it retains its original crystallinity, and has high mechanical strength.
However, the starch nanocrystal itself can not produce fluorescence, thereby restricting its application in the fields of bioimaging marking, biosensor preparation, in vitro and vivo fluorescence imaging, automatic DNA sequencing, etc., and limiting further expansion of the application of starch nanocrystal.
Fluorescein isothiocyanate fails to directly react with the starch nanocrystal, and only trace amount of it is adsorbed, or it cannot be adsorbed on starch nanocrystal. If a fluorophore is introduced into the surface of starch nanocrystal, an active group shall be introduced with the help of a “crosslinking agent” to realize its chemical reaction with fluorescein. A silane coupling agent has two functional groups with different reactivities in its molecule, and can be adsorbed and condensed on the surface of starch nanocrystal, followed by subsequent coupling reaction, thereby introducing active amino groups into the surface of starch nanocrystal. Compared to other intermediate “crosslinking agents”, it has simple reaction and high bonding efficiency, and improves the amount of amino groups introduced into the surface, so that it can further react with fluorescein isothiocyanate efficiently and realize preparation of fluorescent starch nanocrystal.